Our continuing investigation of acetylcholinesterase (AChE), the acetylcholine binding protein (AChBP) reflect a long standing and continuous commitment to the study of proteins that affect the intensity and duration of acetylcholine action, a critical neurotransmitter affecting cognition in the CNS and peripheral autonomic and motor function. Our studies, supported by this grant over the past 37 years, have evolved from considerations of structure and function of AChE and the nicotinic acetylcholine receptor (nAChR) and, more recently, from AChE to a related a/b-hydrolase fold protein, neuroligin. In turn, they have spun off separate drug development endeavors directed novel nicotinic receptor ligands for depression, schizophrenia, pain alleviation and nicotine addiction and AChE inhibitor antidotes. Our fundamental studies with the extracellular domain of the three proteins are now based on structure at atomic resolution, that of static crystal structures, but also extend to an analysis of conformational dynamics and assignments of energetic contributions through structural modification and mutant cycle analysis. Conformation and dynamics are examined through fluorescence spectroscopy, decay of fluorescence anisotropy and H/D exchange. Our nicotinic receptor studies focus on an interfacial site between subunits that can be examined by physical methods through the soluble receptor surrogate, AChBP. Crystal structures of the complexes selected from a wide array of ligands with AChBP enable a detailed analysis of the structural determinants of specificity. We propose to expand to presumed non-competitive sites on this molecule, in particular the vestibule leading into the channel constriction and the non-a subunit interfaces that do not bind agonist. With AChE, we propose to continue our analysis of complexes formed by freeze-frame, click chemistry to compare complexes dictated by kinetics of association and by achieving thermodynamic equilibrium. We also will examine the inductive role of the oxyanion hole, the nucleophile rendering catalytic triad and the hydrogen bonding network between the proximal serine hydroxyls at the base of the gorge. Studies with the heterophilic adhesion protein, neuroligin (NL), capitalize on its homologous structure to AChE, both being members of the a/b-hydrolase-fold family, wherein their common globular domains and unique recognition features help direct the study of NL by low angle scattering and high resolution techniques. This approach is helping to uncover the molecular determinants associated with the interaction of NL with its synaptic partner proteins at its structurally unique binding site. Moreover, the common structural fold between AChE and NL allows comparisons in how mutations affect biosynthesis and trafficking of these two molecules. The shared familial structural features enable us to understand how gene mutations, some of which are associated with autism spectrum disorders, affect the biosynthesis and folding in this protein family.